drm/i915: Enable PSR by default on Valleyview and Cherryview.

With a reliable frontbuffer tracking and all instability corner cases
solved for this platform let's re-enabled PSR by default.

In case a new issue is found and PSR is the main suspect, please check
if i915.enable_psr=0 really makes your problem go away,
please report it at bugs.freedesktop.org.

In a bugzilla entry for PSR is desirable:
- dmesg (drm.debug=0xe)
- output of /sys/kernel/debug/dri/0/i915_edp_psr_status
- Platform information. Vendor, model, id, pci id.
- Graphical environment: Gnome, KDE, openbox, etc...
- Details how to reproduce.
- Also good if you could run PSR test cases of Intel-gpu-tools
- Please mention if forcing main link standby or main link off helps you.

There are Intel-gpu-tools test cases that can be helpful to
determine if PSR is working as expected:
 kms_psr_sink_crc and kms_psr_frontbuffer_tracking.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>